Agents are being sought which localize in the myocardium. When these are labeled with appropriate radionuclides it is possible to image the distribution in the heart and thereby define regions of decreased perfusion. This is of importance in diagnosis of coronary artery disease and myocardial infarction. Thallium-201 is one such agent. Its distribution is similar to potassium but its physical characteristics are superior to available potassium isotopes. C11-norepinephrine localized in nerve endings in the heart to an extent that permits imaging with appropriate instrumentation. Another possibility is a labeled cardiac glycoside.